1. Technical Field
The present invention relates to a method and apparatus for inspecting the density of cigarettes, and more particularly, to a cigarette density inspection method and apparatus for detecting a local low-density portion of cigarettes.
2. Related Art
The quality of cigarettes may be impaired, if the filling density of tobacco shred in cigarettes (hereinafter, referred to as cigarette density) is nonuniform. For instance, if a cigarette includes a local low-density portion which expedites the burning of tobacco shreds, the flavor of smoking is degraded since an amount of air sucked by a smoker changes at the low-density portion.
One of the causes of the presence of a local low-density portion in cigarettes resides in that so-called puff tobacco leaves expanded by a treatment using carbon dioxide gas or the like are frequently used for the production of cigarettes. Tobacco shreds including puff tobacco are liable to be nonuniformly formed into a cigarette, so that nonuniformity occurs in the cigarette density. Another cause is that recent cigarette manufacturing machines are configured to wrap tobacco shreds by a paper web at a faster speed. The high wrapping speed makes it difficult to uniformly form cigarettes.
In the cigarette manufacturing process, cigarettes including a local low-density portion longer than a predetermined length are generally rejected as being faulty products. To this end, a density detector is mounted on a cigarette manufacturing machine, and, in accordance with the density signal generated by the density detector, inspection is made to determine whether a tobacco rod continuously produced by the cigarette manufacturing machine includes such a local low-density portion.
As disclosed in U.S. Pat. Nos. 2,954,775, 2,937,280, 3,056,026 and Japanese patent publication no. 8-2288, various density detectors are known, which utilize a relationship between cigarette density and penetration of beta radiation, X-ray radiation, infrared radiation or ultrasonic wave or between cigarette density and permittivity. Among these detectors, a beta radiation type density detector is mainly employed for the reason that it is largely unaffected by moisture content, tobacco-shred size and color of measured cigarettes, and is hence excellent in measurement stability, as compared to other types of density detectors.
Even in the case of using a radiation type density detector, however, a difficulty is encountered if an attempt is made to further improve the measurement accuracy. To accurately detect a local low-density portion in cigarettes, it is advisable to narrow the width of a window which is formed in a detecting section of the density detector and through which beta radiation passes. For instance, the window width may be narrowed to a value equivalent to the minimum length (e.g., 3-4 mm) of those local low-density portions which may cause the quality of cigarettes to be lowered to an extent that they are rejected as being faulty products.
If, however, the window width is narrowed to such an extent, the quantity of beta radiation passing through the window becomes smaller, lowering the measurement reliability. That is, the quantity of beta radiation generated by strontium-90, which is radioactive isotope and which collapses randomly, varies to form a binominal distribution, so that the standard deviation of a density signal obtained by measuring the quantity of beta radiation passing through the window is inversely proportional to the square root of the quantity of beta radiation passing through the window. In other words, the smaller the quantity of beta radiation becomes, the lower the reliability of density signal will be.
In order to simply increase the quantity of beta radiation passing through the window of a radiation type density detector, it is sufficient to use a detector which contains a larger amount of strontium-90. In this case, however, an amount of leakage of braking radiation increases.
If the quantity of beta radiation is small, the gain of an amplifier which constitutes part of a density detector must be increased, and a drift in the amplifier output is liable to increase. In this case, the density signal may be affected by the drift of the amplifier output, causing an error in the measurement, based on the density signal, of average cigarette weight which is important factor for management of cigarette quality.
To attach the importance to the accuracy of average cigarette weight measurement, the window width is usually set to a value equivalent to cigarette diameter, e.g., about 8mm, as disclosed in U.S. Pat. 3,056,026, although it is apparent that the accuracy of detection of a low-density cigarette portion can be improved by using a density detector which is narrow in window width.
However, it is difficult to detect a local low-density portion on the basis of the output from a radiation type density detector which is so configured to generate the output suitable to measure the average cigarette weight. More specifically, if a density detector, having the window width equivalent to the cigarette diameter, is used to detect a local low-density portion which is approximately 3-4 mm in length and which has a weight about 65-70% lighter than that of a standard density portion, the waveform of the density signal is distorted because of the window width which is longer than the length of the local low-density portion. That is, even if the low-density portion, having a density which is discontinuous from that of a standard-density portion, passes the window of the density detector, the density signal merely changes in a manner describing a gentle valley. In addition, a change in density signal level observed between when the standard-density portion passes the detector and when the low-density portion passes the same is about half of the density difference between these two portions. Namely, only 20% drop occurs in signal level even for a low-density portion whose density is 40% smaller than that of the standard-density portion.
A variation (standard deviation) in weight in the axial direction of a cigarette having 8 mm diameter is generally in the order of 8%. Thus, a variation of about .+-.24% may occur in an ordinary signal level, if the window width is widened, attaching to the importance to the accuracy of average weight detection, up to a value of 8 mm which is about three times larger than the length (approximately 3 mm) of local low-density portion to be detected. Thus, a change in signal level in the order of 20% indicative of the presence of local low-density portion cannot be distinguished from the ordinary variation of about 24% in signal level.
As a consequence, it is difficult to appropriately carry out both the measurement of average cigarette weight and the detection of local low-density portion, based on the density signal obtained by a radiation type density detector formed with a window which is large in width. This is also true of to density detectors of types other than the radiation type.